This invention relates generally to optical filtering and, more particularly, to optical filtering for chemical identification such as gas detection.
It is well known to detect, measure and/or analyze the characteristics of a fluid mixture, such as composed of a gas, liquid or mixture thereof, via evaluation of the absorption spectra obtained via optical methods. Birefringent filters have been used for a variety of diverse applications, including emission spectrum filtering of sodium doublet D lines and electro-optical modulators. The basic operation of such devices is described in various textbooks including Amnon Yariv, Optical Electronics, 3rd Edition, Holt, Rinehart and Winston, chapter 9, (1985) and Eugene Hecht and Alfred Zajac, Optics, Addison-Wesley Publishing Company, Reading, Mass., pp 263-266, (1974), for example and the disclosures of which are hereby incorporated in their entirety.
Typically, light is collimated prior to being projected through a birefringent filter to ensure a uniform phase modulation across the aperture of the incoming beam. A problem with using birefringent filters in collimated light is that the collimating process can be inefficient in collecting the light thus making it difficult to produce corresponding assemblies or devices having relatively low power consumption requirements, such as may be suitable or desired in particular applications such as for portable or battery-powered gas detection instruments.
In view of the above, there is a need and a demand for improved gas detection assemblies and methods such as permit or allow for a more efficient use of light. Further, there is a need and a demand for improved gas detection assemblies and methods such as eliminate the need for collimating optics and allows for a higher throughput of light while reducing or minimizing the number of required optical elements.
A general object of the invention is to provide an improved assembly and method for detecting a target species in a selected sample.
A more specific objective of the invention is to overcome one or more of the problems described above.
The general object of the invention can be attained, at least in part, through an assembly which includes a source of convergent electromagnetic radiation spaced apart from a tunable birefringent filter by a sample volume. The assembly also includes at least one detector to detect the incidence of electromagnetic radiation of a selected wavelength after passage of convergent electromagnetic radiation through the sample volume and the birefringent filter.
The prior art generally fails to provide suitably low power consuming assemblies and methods such as for use in the detection of a target species in a test sample. Further, the prior art has generally failed to provide such detection assemblies and methods having either or both a construction or an operation which is as simple as may be desired. Consequently, the prior at has generally failed to provide such detection assemblies and methods which are as conducive to portable operation and use as has been desired.
The invention further comprehends an improvement in a method for detecting the presence of a target species in a sample volume wherein light is passed through the sample volume to a birefringent filter. In accordance with a preferred embodiment of the invention, the improvement of the invention involves passing convergent light rays through the sample volume to the birefringent filter.
The invention still further comprehends a method for detecting the presence of a target species in a gas sample, wherein the target species absorbs light in a predetermined spectral region. In accordance with one preferred embodiment of the invention, such a method involves passing convergent light through the gas sample to a birefringent filter and subsequently to a detector. The birefringent filter is tuned through the predetermined spectral region and a signal is produced representing the presence of the target species in the gas sample.
As used herein, references to a xe2x80x9cconvergent birefringent filterxe2x80x9d or the like are to be understood to refer to a birefringent filter in or through which noncollimated, preferably convergent, light is passed.
Other objects and advantages will be apparent to those skilled in the art from the following detailed description taken in conjunction with the appended claims and drawings.